In the case where such a valve, for example, a gate valve or butterfly valve, is used, there is sometimes adopted a mechanism for opening and closing the valve, wherein a rectilinear motion of a piston operated by an air pressure or oil pressure is converted into a rotary motion, and by this rotary motion a valve body is shifted relative to a valve seat to open and close the valve.
In that case, it is known that the rectilinear motion may be converted into the rotary motion, for example, by a mechanism wherein the piston that moves rectilinearly is provided with a rack gear, a member that rotates is provided with a pinion gear mating with this rack gear, and the valve is opened or closed when the pinion gear is rotated by the rectilinear motion of the rack gear.
In this rack-and-pinion mechanism, however, that is such a drawback that because the center of rotation of the pinion gear is perpendicular to the rectilinear shifting direction of the rack gear, the mechanism is liable to become large in size. In addition, the valve using such a mechanism as mentioned above has such a drawback that in order to maintain sealing properties between the portion at which the valve body contacting with a fluid is located and the portion that drives the valve body to shift, a sliding material must be fitted to the sliding portion, but the sliding portion to which the sliding material has been applied is apt to be insufficient in sealing properties.
Further, there is developed a valve operating mechanism which is designed that the traveling direction of a rectilinear traveling means is in the same direction as in the center of rotation of a rotary traveling means. For example, in a valve operating mechanism comprising a cylindrical body having a convex portion on its inner periphery as a rectilinear traveling means and a rotary shaft having a helical groove mating with said convex portion as a rotary traveling means, said rotary shaft being fitted inside said cylindrical body, the rotary shaft used as the rotary traveling means is rotated by traveling rectilinearly the cylindrical body, while mating the convex portion with the helical groove.
In the mechanism as mentioned above, however, the convex portion is difficult to move through the helical groove and, moreover, a large frictional force is produced between the convex portion and the helical groove. On that account, an efficiency of converting a rectilinear motion into a rotary motion is poor, and a loss of energy arising from this conversion must be supplemented anyhow, hence a force larger than the predetermined force should be applied to this cylindrical body used as the rectilinear traveling means. Accordingly, there was such a problem that because such a large force as mentioned above is needed, the mechanism as illustrated must be unavoidably large in size.
In addition thereto, when a vacuum valve is operated, for example, by means of the above-mentioned valve operating mechanism, the main valve chamber wherein the valve body travels must be evacuated, while the valve operating mechanism is in an atmosphere. On that account, as a sealing material used between the main valve chamber and the fixed side of the valve operating mechanism, there is provided a bellows flexible with the traveling of the cylindrical body so that the bellows covers the rotary shaft used as the rotary traveling means. However, this mechanism has such a problem that a rotational force of the rotary shaft is sometimes applied to the bellows which then undergo deterioration with time to a marked extent, thus the service life of the bellows is shortened. Furthermore, there is another problem that because a force larger than necessary is needed, a large-sized valve operating mechanism has come to be required, with the result that the borrows of large size is needed.